Antennas are utilized for communication. Aircrafts rely on communication. Special considerations are considered when utilizing an antenna on an aircraft and/or other moving vehicles and devices.
Monopole antennas are typically vertically polarized antenna structure. Monopole antennas can be thought of as replacing one half of a dipole antenna with a ground plane at a right-angle to another half. Monopole antennas are typically large and/or non-aerodynamic. A dipole antenna may have two conductors pointed in opposite directions with one end of a conductor attached to a radio and one end of the other conductor hanging free in space. A dipole and a monopole antenna may be positioned vertically or horizontally and are omnidirectional in azimuth with a low gain. Both monopole and dipole antennas are inherently narrowband since they are fundamentally resonant structures.
Broadband multi-arm spiral antennas provide an omni type radiation patterns over a wide bandwidth. Broadband multi-arm spiral antennas require complicated BALUN networks (e.g., a passive electrical device that converts between balanced and unbalanced radio frequency electrical signals). Moreover, broadband multi-arm spiral antennas typically have large diameters and no capability for dynamic impedance tuning for an electronically scanned arrays (ESA).